Multiprotocol Label Switching (MPLS)

1. Multiprotocol Label Switching(MPLS) 中正大學資工系 黃仁竑

2. Outline • Introduction • Label Encoding • Label Assignment • Label Distribution • Label Swapping • Label Merging • Conclusion 中正資工/黃仁竑

3. Introduction • IETF Multiprotocol Label Switching Working Group created in 1997 • Integrates the label swapping forwarding paradigm with network layer routing • Use a short, fixed-length label • Examples of MPLS • Tag Switching (Cisco) 中正資工/黃仁竑

4. Why MPLS • MPLS versus Datagram Routed Network • Simplified forwarding • Efficient explicit routing • Traffic engineering • QoS routing • Complex mapping from IP packets to FEC (Forwarding Equivalence Class) • MPLS versus ATM • Scaling of the routing protocol • Common operation over packet and cell media • Easier management 中正資工/黃仁竑

5. Basics of MPLS • Semantics assigned to a stream label • Labels are associated with specific streams of data (FEC) • Forwarding Methods • Short fixed length labels to identify streams • Looking up a label in a table, swapping labels, and possibly decreasing and checking a TTL • May make direct use of layer 2 forwarding (e.g. ATM) • Label Distribution Methods • Allow nodes to determine which labels to use for specific streams • May use some sort of control exchange, or be piggybacked on a routing protocol • By LDP 中正資工/黃仁竑

6. Next Hop Label Forwarding Entry • NHLFE is used when forwarding a labeled packet, it contains • next hop • operations • replace the label at the top of the label stack with a new label • pop the label stack • replace and push one or more new labels • data link encapsulation • how to encode the label stack • other information for properly dispose of the packet 中正資工/黃仁竑

7. Label Stack • Label stack • A labeled packet may carry a number of labels • A Label • A short fixed length significant identifier • Based on the stream or forwarding equivalence class (FEC) • Only have local significance • Label encoding • MPLS generic encapsulation mechanism • ATM SVC, SVP, SVP multipoint encoding methods • Others 中正資工/黃仁竑

8. Label Switched Path (LSP) • Begins with an LSR (LSP Ingress) that pushes on a level m label • Intermediate LSRs make their forwarding decision by label switching on a level m label • Ends (LSP Egress) when forwarding decision is made by label switching on a level m-k label (k>0) or when a forwarding decision is made by non-MPLS forwarding procedures • The label stack may be popped at the penultimate LSR of the LSP, rather than at the LSP Egress • reduce times of label lookup at LSP egress 中正資工/黃仁竑

9. Label Encoding • Generic MPLS encapsulation • Between the data link layer and network layer headers • Network layer protocol independent • A label contains • Label Stack • A sequence of label stack entries • Time-to-Live (TTL) • Similar to what is provided by IP (e.g. traceroute) • Class of Service (CoS) • Allows multiple service classes within the same label 中正資工/黃仁竑

10. 0 20 23 31 Label Exp S TTL Label Stack Entry • Label(20bits) • carries the actual value of the label • 0/2:IPv4/v6 Explicit NULL Label • must be sole label stack entry (forward based on IPv4/v6) • 1:Router Alert Label;(need software process) • 3:Implicit NULL Label • Exp(3bits):reserved • S(1bits):Bottom of Stack • TTL(8bits):Time to Live 中正資工/黃仁竑

11. Label Encoding • ATM Switches as LSRs • SVC Encoding • Use the VPI/VCI field to encode the label • Each LSP is realized as an ATM SVC • ATM-LSR cannot perform PUSH or POP • SVP Encoding • VPI : Top of label stack • VCI : Second label on the stack • Permits the use of ATM VP switching • can’t include a non-MPLS ATM network • SVP Multipoint Encoding • VPI : Top of label stack • VCI : Part for the second label on the stack, the remainder to identify the LSP ingress • Multipoint-to-point VPs 中正資工/黃仁竑

12. Label Assignment • Topology driven (Tag) • In response to normal processing of routing protocol control traffic • Labels are pre-assigned; no label setup latency at forwarding time • Request driven (RSVP) • In response to normal processing of request based control traffic • May require a large number of labels to be assigned • Traffic driven (Ipsilon) • The arrival of data at an LSR triggers label assignment and distribution • Label setup latency; potential for packet reordering 中正資工/黃仁竑

13. Label Distribution • Explicit Label Distribution • Downstream label allocation • label allocation is done by the downstream LSR • most natural mechanism for unicast traffic • Upstream label allocation • label allocation is done by the upstream LSR • may be used for optimality for some multicast traffic • A unique label for an egress LSR within the MPLS domain • Any stream to a particular MPLS egress node could use the label of that node. 中正資工/黃仁竑

14. Label Distribution • Explicit Label Distribution Protocol (LDP) • Reliability : by transport protocol (TCP) or as part of LDP • Separate routing computation and label distribution • Piggybacking on Other Control Messages • Use existing routing/control protocol for distributing routing/control and label information • OSPF, BGP, RSVP, PIM • Combine routing and label distribution • Label purge mechanisms • By time out • Exchange of MPLS control packets 中正資工/黃仁竑

15. Label Distribution Protocol • LDP Peer: • Two LSRs that exchange label/stream mapping information via LDP • LDP messages • Discovery messages • announce and maintain the presence of LSR • via UDP • Session messages • maintain session between LDP peers • Advertisement message • label operation (Label distribution) • Notification message • advisory information and signal error information • Error notification:signal fatal errors • Advisory notification: status of the LDP session or some previous message received from the peer. 中正資工/黃仁竑

16. Label Swapping • Labeled Packet • Map the incoming label to an next hop label, determines where to forward the packet • Encodes the new label stack into the packet, and then forwards it • Unlabeled Packet • LSR analyzes the L3 header, to determine the packet’s stream • Map the stream to a next hop, determines where to forward the packet • Encodes the new label stack into the packet, and then forwards it 中正資工/黃仁竑

17. Use of MPLS in a Hierarchy 中正資工/黃仁竑

18. Route Selection • Hop by hop routing • Like conventional IP routing • Each hop makes independent choice of next hop • Repair of a failed route done locally • Explicit routing • Manual or based on dynamic routing • The LSP next hop is chosen by a single node • Useful for policy routing and/or traffic engineering • If an explicit route is specified for an LSP, then that route must be followed 中正資工/黃仁竑

19. Loop Handling • Loop Survival • minimizes the impact of loops • Loop Detection • allows loops to be set up, but detects them and eliminates them later • Loop Prevention • avoiding setting up a loop 中正資工/黃仁竑

20. Loop Survival • Allow the network to operate well even though short term transient loops may be formed by the routing protocol • Possible solutions • Use of TTL to limit the hops that a packet traversed • Use of dynamic routing protocol which converges fast • looping packets may cause congestion which may then affect the converge speed of routing protocol • Use of fair queueing to limit the impact of looping packets on normal packets 中正資工/黃仁竑

21. Loop Detection • Loop may be set up, but will be subsequently detected. • Possible solutions • Loop Detection Control Protocol (LDCP) • transmit LDCP packet when route change • LDCP is forwarded towards destination until • destination • TTL exceeded • return to a node which originally transmitted it • Path Vector Control message: list of LSRs on the path • hop count to each egress node (like RIP?) 中正資工/黃仁竑

22. Loop Prevention • Ensure loops are never set up • Possible solutions • labels are propagated from the egress switch, control packets which propagate the labels also include the path • diffusion mechanism when route changes • colored mechanism • a color consist of address of the node that created the color and a local id that is unique within the node • a node that finds a change in the next hop creates a color and passes it to the new next hop • stops when a loop or a loop free path is found • explicit routing • configured • use routing protocol (link state or path vector) 中正資工/黃仁竑

23. Diffusion Algorithm • On a route change, R ask N for a label and the associated LSR ID for that stream • R looks in the LSR ID list • If R is in the list (route loop), the old LSP will continue to be used until the route protocol break the loop • If R is not in the list, R will start a diffusion computation • Diffusion computation prunes tree of paths that would loop if R switches to new LSP • When the diffusion completes, R switches to new LSP and discards old LSP 中正資工/黃仁竑

24. Diffusion Computation • An extension of Path Vector mechanism • An LSR, D, detects the next hop for an FEC has changed, transmits a query message with a Path Vector containing its id to its upstream • A LSR, U, that receives a query will determine if D is the next hop for the given FEC • if not, then U return OK message • if so, then U checks if the Path Vector already contains it id • if yes, a loop is detected, U responds with a LOOP msg • if not, U adds its id to the Path Vector and propagates the query message to its upstream neighbors 中正資工/黃仁竑

25. What to Do if a Loop is Detected • If a loop is know to exist • L2 label-swapped path is not setup • Packet is forwarding using normal L3 forwarding • Problems : • Nodes which are not capable of L3 forwarding • discard packet • L2 forwarding faster than L3 forwarding • node will not be capable of forwarding the same volume of traffic at l3, some packets will be discarded • packet lost cause TCP to backoff, which will in turn reduce the load and allow the network to stabilize until the label binding is reestablished again. 中正資工/黃仁竑

26. Label Merging • Label merging • An LSR may want to bind multiple incoming labels to a particular FEC • once packets are transmitted, the information that they arrived with different labels is not • Non-merging LSRs • In ATM, label merging may cause interleaving of cells from various packets • MPLS support procedures which allow ATM switches to function as merging LSRs 中正資工/黃仁竑

27. Merge over ATM • VP Merge (SVP multipoint encoding) • Packet from different sources are distinguished by using different VCs within the VP • Advantage : no new hardware • Disadvantage : requires coordination of the VCI space • VC Merge • Switches are required to buffer cells from one packet until the entire packet is received • Advantage : straightforward application of VC switching • Disadvantage : • New hardware (based on per-VC queuing) • Delays at the merge points 中正資工/黃仁竑

28. Conclusion • MPLS • A more general forwarding mechanism • Cooperates with routing/control protocol • Provides Integrated service, Differentiated service • Allows flow aggregation (FEC) for QoS routing • Support Multicast? 中正資工/黃仁竑